Systems and methods for annealing semiconductor structures

ABSTRACT

Systems and methods are provided for annealing a semiconductor structure. For example, a semiconductor structure is provided. An energy-converting material capable of increasing the semiconductor structure&#39;s absorption of microwave radiation is provided. A heat reflector is provided between the energy-converting material and the semiconductor structure, the heat reflector being capable of reflecting thermal radiation from the semiconductor structure. Microwave radiation is applied to the energy-converting material and the semiconductor structure to anneal the semiconductor structure for fabricating semiconductor devices.

FIELD

The technology described in this patent document relates generally tosemiconductor materials and more particularly to processing ofsemiconductor materials.

BACKGROUND

Modern semiconductor devices are often fabricated through manyprocesses. For example, a semiconductor substrate for device fabricationmay be doped (e.g., adding desired impurities into the substrate) toform junctions. Dopants introduced into the substrate are usuallyelectrically activated before semiconductor devices can be fabricated onthe substrate. The activation of the dopants often includes transferringthe dopant atoms/molecules from interstitial positions into latticesites of the lattice structure of the substrate. Different annealingtechniques may be used for dopant activation, such as rapid thermalannealing (RTA), and laser annealing.

Under certain circumstances, the fabrication process of semiconductordevices involves microwave radiation which typically includeselectromagnetic waves with wavelengths ranging from 1 m to 1 mm(corresponding to frequencies between 0.3 and 300 GHz). When microwaveradiation is applied to a certain material (e.g., a dielectric material)which includes electric dipoles, the dipoles change their orientationsin response to the changing electric fields of the microwave radiationand thus the material may absorb the microwave radiation to generateheat. The response of the material to the electric field of themicrowave radiation can be measured using a complex permittivity, ∈(ω)*,which depends on the frequency of the electric field:∈(ω)*=∈(ω)′−i∈(ω)″=∈₀(∈_(r)(ω)′−i∈ _(r)(ω)″)  (1)where ω represents the frequency of the electric field, ∈(ω)′ representsa real component of the complex permittivity (i.e., a dielectricconstant), and ∈(ω)″ represents a dielectric loss factor. In addition,∈₀ represents the permittivity of a vacuum, ∈_(r)(ω)′ represents therelative dielectric constant, and ∈_(r)(ω)″ represents the relativedielectric loss factor.

Whether a material can absorb the microwave radiation can becharacterized using a loss tangent, tan δ:

$\begin{matrix}{{\tan\;\delta} = \frac{{ɛ^{''}\mu^{\prime}} - {ɛ^{\prime}\mu^{''}}}{{ɛ^{\prime}\mu^{\prime}} + {ɛ^{''}\mu^{''}}}} & (2)\end{matrix}$where μ′ represents a real component of the magnetic permeability of thematerial, and μ″ represents a magnetic loss factor. Assuming negligiblemagnetic loss (i.e., μ″=0), the loss tangent of a material is expressedas follows:

$\begin{matrix}{{\tan\;\delta} = {\frac{ɛ^{''}}{ɛ^{\prime}} = \frac{ɛ_{r}^{''}}{ɛ_{r}^{\prime}}}} & (3)\end{matrix}$

Materials with a low loss tangent (e.g., tan δ<0.01) allow microwaves topass through with very little absorption. Materials with an extremelyhigh loss tangent (e.g., tan δ>10) reflect microwaves with littleabsorption. Materials with an intermediate loss tangent (e.g., 10≧tanδ≧0.01) can absorb microwave radiation.

SUMMARY

In accordance with the teachings described herein, systems and methodsare provided for annealing a semiconductor structure. For example, asemiconductor structure is provided. An energy-converting materialcapable of increasing the semiconductor structure's absorption ofmicrowave radiation is provided. A heat reflector is provided betweenthe energy-converting material and the semiconductor structure, the heatreflector being capable of reflecting thermal radiation from thesemiconductor structure. Microwave radiation is applied to theenergy-converting material and the semiconductor structure to anneal thesemiconductor structure for fabricating semiconductor devices.

In one embodiment, a system for annealing a semiconductor structureincludes an energy-converting material, a heat reflector, and amicrowave-radiation source. The energy-converting material is configuredto increase a semiconductor structure's absorption of microwaveradiation. The heat reflector is configured to reflect thermal radiationfrom the semiconductor structure, the heat reflector being disposedbetween the energy-converting material and the semiconductor structure.The microwave-radiation source is configured to apply microwaveradiation to the energy-converting material and the semiconductorstructure to anneal the semiconductor structure for fabricatingsemiconductor devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an example diagram for annealing a semiconductorstructure using microwave radiation.

FIG. 2 depicts another example diagram for annealing a semiconductorstructure using microwave radiation.

FIG. 3 depicts yet another example diagram for annealing a semiconductorstructure using microwave radiation.

FIG. 4 depicts another example diagram for annealing a semiconductorstructure using microwave radiation.

FIG. 5 depicts an example diagram showing a system for annealing asemiconductor structure using microwave radiation.

FIG. 6 depicts an example diagram showing a flow chart for annealing asemiconductor structure using microwave radiation.

DETAILED DESCRIPTION

The conventional annealing technologies often have some disadvantages.For example, a substrate used for device fabrication often includesvarious device patterns (e.g., through deposition, lithography and/oretching). These different patterns usually correspond to differentthicknesses and material types which result in different heatemissivity. During an annealing process (e.g., RTA), different regionson the substrate often absorb and emit different amounts of heat, whichresults in localized temperature non-uniformity on the substrate.Furthermore, photons of light sources (e.g., lamps used for RTA orlasers used for laser annealing) may not penetrate beyond surfaceregions of the substrate, which often causes uneven heating of thesubstrate at different depths.

FIG. 1 depicts an example diagram for annealing a semiconductorstructure using microwave radiation. As shown in FIG. 1, microwaveradiation is applied to an energy-converting material 102 and asemiconductor structure 104 to anneal the semiconductor structure 104(e.g., for dopant activation). A heat reflector 106 is disposed betweenthe energy-converting material 102 and the semiconductor structure 104to reflect thermal radiation back to the semiconductor structure 104during the annealing process. Microwave radiation penetrates deeply intothe semiconductor structure 104 and results in volumetric heating of thesemiconductor structure 104.

Specifically, the semiconductor structure 104 which has a small losstangent may not absorb microwave radiation efficiently. Theenergy-converting material 102 which has a large loss tangent (e.g., ina range of about 0.01 to about 2) absorbs microwave radiation moreefficiently, and in response a temperature associated with theenergy-converting material 102 increases rapidly. On one hand, the heatgenerated by the energy-converting material 102 may increase thetemperature of the semiconductor structure 104 (e.g., through conductionor radiation). At the elevated temperature, the interaction between thesemiconductor structure 104 and the microwave radiation increases. Onthe other hand, the energy-converting material 102 increases an electricfield density over the semiconductor structure 104 (e.g., thetemperature of the semiconductor structure 104 being raised to 400°C.-700° C.). For example, at the raised electric field density, theelectric field reacts with defects within the semiconductor structure104 through interfacial polarization so as to further activate thedopants.

During the annealing process, the semiconductor structure 104 emitsthermal radiation (e.g., infrared radiation). The heated heat reflector106 reflects such thermal radiation back to the semiconductor structure104 so that the thermal radiation generated by the semiconductorstructure 104 is conserved between the heat reflector 106 and thesemiconductor structure 104 (e.g., to reduce localized temperaturenon-uniformity). The distance (e.g., d1) between the heat reflector 106and the semiconductor structure 104 may be adjusted to ensure that mostthermal radiation from the semiconductor structure 104 is reflectedback. In certain embodiments, the heat reflector 106 is placed on top ofdevice patterns formed on the semiconductor structure 104. For example,the heat reflector 106 is approximately transparent to the microwaveradiation, and is opaque to the thermal radiation (e.g., infraredradiation). As an example, the heat reflector 106 has a reflectivitylarger than 95% with respect to infrared radiation. In some embodiments,the heat reflector 106 includes a semiconductor wafer (e.g., a blanksilicon wafer) that contains dopants to provide additional freecarriers. In certain embodiments, the heat reflector 106 includes asemiconductor wafer (e.g., a blank silicon wafer) that does not containdopants.

For example, the semiconductor structure 104 includes a junction with anumber of dopants formed on a substrate at an elevated temperature(e.g., in a range of about 300° C. to about 600° C.) by epitaxialgrowth. The microwave radiation is applied to anneal the semiconductorstructure 104 for dopant activation. The energy-converting material 102intensifies the electric field density over the semiconductor structure104. The semiconductor structure 104 absorbs more microwave radiationunder the increased electric field density. More and more reactionsbetween the electric field and defects in the semiconductor structure104 occur through interfacial polarization. For example, positivecharges and/or negative charges accumulated at boundaries of the defectsmay cause the defects to dissolve eventually for further dopantactivation. Once the electric field density over the semiconductorstructure 104 exceeds a threshold, the interfacial polarizationeventually breaks the bonds between the dopants and the interstitialsites in the semiconductor structure 104, so that the dopants areactivated. The distance (e.g., d2) between the energy-convertingmaterial 102 and the semiconductor structure 104 may be adjusted toimprove the dopant activation. For example, the dopants includephosphorous, phosphorous-based molecules, germanium, helium, boron,boron-based molecules, or other materials.

In some embodiments, the microwave radiation applied to theenergy-converting material 102 has a frequency in the range of about 2GHz to about 10 GHz. For example, the energy-converting material 102includes boron-doped silicon germanium, silicon phosphide, titanium,nickel, silicon nitride, silicon dioxide, silicon carbide, n-type dopedsilicon, aluminum cap silicon carbide, or other materials. Theenergy-converting material 102 may have a larger size than thesemiconductor structure 104 so that the electric field density may beapproximately uniform over the semiconductor structure 104. In certainembodiments, the temperature of the semiconductor structure 104 is keptwithin a range of about 300° C. to about 600° C. to reduce dopantdiffusion. For example, the microwave radiation is applied to theenergy-converting material 102 and the semiconductor structure 104 for atime period within a range of about 40 seconds to about 300 seconds.

FIG. 2 depicts another example diagram for annealing a semiconductorstructure using microwave radiation. As shown in FIG. 2, microwaveradiation is applied to an energy-converting material 202 and asemiconductor structure 204 to anneal the semiconductor structure 204(e.g., for dopant activation). A heat reflector 206 is formed on theenergy-converting material 202 and is disposed on top of thesemiconductor structure 204 to reflect thermal radiation back to thesemiconductor structure 204 during the annealing process. For example,the heat reflector 206 is formed on the energy-converting material 202through epitaxial growth, (e.g., deposition). As an example, theenergy-converting material 202 is capable of generating the thermalradiation in response to the applied microwave radiation and increasingan electric field density over the semiconductor structure 204.

During the annealing process, the heat reflector 206 reflects thermalradiation emitted by the semiconductor structure 204 back to thesemiconductor structure 204 so that such thermal radiation is conservedbetween the heat reflector 206 and the semiconductor structure 204(e.g., to reduce localized temperature non-uniformity). For example, theheat reflector 206 (e.g., a blank silicon wafer) is approximatelytransparent to the microwave radiation and opaque to the thermalradiation (e.g., infrared radiation).

FIG. 3 depicts yet another example diagram for annealing a semiconductorstructure using microwave radiation. As shown in FIG. 3, microwaveradiation is applied to two energy-converting materials 302 and 308 anda semiconductor structure 304 to anneal the semiconductor structure 304(e.g., for dopant activation). A heat reflector 306 is disposed betweenthe energy-converting material 302 and the semiconductor structure 304to reflect thermal radiation back to the semiconductor structure 304during the annealing process.

For example, the energy-converting materials 302 and 308 have a sameloss tangent or different loss tangents. As an example, the distance(e.g., d1) between the energy-converting material 302 and thesemiconductor structure 304 is the same as or different from thedistance (e.g., d2) between the energy-converting material 308 and thesemiconductor structure 304. In some embodiments, the heat reflector 306is formed on the energy-converting material 302 through epitaxialgrowth, (e.g., deposition), as shown in FIG. 4.

FIG. 5 depicts an example diagram showing a system for annealing asemiconductor structure using microwave radiation. As shown in FIG. 5, ashell 502 includes a microwave port 504 through which microwaveradiation may be introduced into the shell 502. During the annealingprocess, microwave radiation generated by a microwave-radiation source512 is applied to an energy-converting material 506 and a semiconductorstructure 510 to anneal the semiconductor structure 510 (e.g., fordopant activation). A heat reflector 508 is disposed between theenergy-converting material 506 and the semiconductor structure 510 toreflect thermal radiation back to the semiconductor structure 510. Forexample, the shell 502 is made of a metal material. As an example, theenergy-converting material 506 is pre-heated to a predeterminedtemperature (e.g., in a range of about 300° C. to about 600° C.) by aheat source (e.g., an Ar lamp, a Xeon lamp, or a tungsten-halogen lamp).

In some embodiments, an additional energy-converting material is placedin the shell 502 so that the semiconductor structure 510 is disposedbetween the heat reflector 508 and the additional energy-convertingmaterial. In certain embodiments, the heat reflector 508 is formed onthe energy-converting material 506 through epitaxial growth.

FIG. 6 depicts an example diagram showing a flow chart for annealing asemiconductor structure using microwave radiation. At 602, asemiconductor structure is provided. For example, the semiconductorstructure includes a plurality of dopants. At 604, an energy-convertingmaterial capable of increasing the semiconductor structure's absorptionof microwave radiation is provided. For example, the energy-convertingmaterial is capable of increasing an electric field density associatedwith the semiconductor structure. As an example, the energy-convertingmaterial includes boron-doped silicon germanium, silicon phosphide,titanium, nickel, silicon nitride, silicon dioxide, silicon carbide, orother materials.

At 606, a heat reflector is disposed between the energy-convertingmaterial and the semiconductor structure. The heat reflector is capableof reflecting thermal radiation from the semiconductor structure. Forexample, the heat reflector includes a blank silicon wafer. The thermalradiation includes infrared radiation. In some embodiments, the heatreflector has a reflectivity larger than 95% with respect to infraredradiation, and is approximately transparent to the microwave radiation.

At 608, microwave radiation is applied to the energy-converting materialand the semiconductor structure to anneal the semiconductor structure(e.g., for dopant activation). For example, the energy-convertingmaterial increases the electric field density in response to themicrowave radiation so as to affect dipoles formed in the semiconductorstructure and motions of the formed dipoles. The formed dipoles arerelated to dopants in the semiconductor structure. As an example, thesemiconductor structure's absorption of microwave radiation is increasedin response to the increased electric field density so as to dissolvecertain dopant clusters included in the semiconductor structure.

This written description uses examples to disclose embodiments of thedisclosure, include the best mode, and also to enable a person ofordinary skill in the art to make and use various embodiments of thedisclosure. The patentable scope of the disclosure may include otherexamples that occur to those of ordinary skill in the art. One ofordinary skill in the relevant art will recognize that the variousembodiments may be practiced without one or more of the specificdetails, or with other replacement and/or additional methods, materials,or components. Further, persons of ordinary skill in the art willrecognize various equivalent combinations and substitutions for variouscomponents shown in the figures.

Well-known structures, materials, or operations may not be shown ordescribed in detail to avoid obscuring aspects of various embodiments ofthe disclosure. Various embodiments shown in the figures areillustrative example representations and are not necessarily drawn toscale. Particular features, structures, materials, or characteristicsmay be combined in any suitable manner in one or more embodiments. Thepresent disclosure may repeat reference numerals and/or letters in thevarious examples, and this repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed. Various additionallayers and/or structures may be included and/or described features maybe omitted in other embodiments. Various operations may be described asmultiple discrete operations in turn, in a manner that is most helpfulin understanding the disclosure. However, the order of descriptionshould not be construed as to imply that these operations arenecessarily order dependent. In particular, these operations need not beperformed in the order of presentation. Operations described herein maybe performed in a different order, in series or in parallel, than thedescribed embodiments. Various additional operations may be performedand/or described. Operations may be omitted in additional embodiments.

This written description and the following claims may include terms,such as top, on, over, etc. that are used for descriptive purposes onlyand are not to be construed as limiting. The embodiments of a device orarticle described herein can be manufactured, used, or shipped in anumber of positions and orientations. For example, terms designatingrelative vertical position may refer to a situation where a device side(or active surface) of a substrate or integrated circuit is the “top”surface of that substrate; the substrate may actually be in anyorientation so that a “top” side of a substrate may be lower than the“bottom” side in a standard terrestrial frame of reference and may stillfall within the meaning of the term “top.” The term “on” or “over” asused herein (including in the claims) may not necessarily indicate thata first layer/structure “on” or “over” a second layer/structure isdirectly on or over and in immediate contact with the secondlayer/structure unless such is specifically stated; there may be one ormore third layers/structures between the first layer/structure and thesecond layer/structure. The term “substrate” used herein (including inthe claims) may refer to any construction comprising one or moresemiconductive materials, including, but not limited to, bulksemiconductive materials such as a semiconductive wafer (either alone orin assemblies comprising other materials thereon), and semiconductivematerial layers (either alone or in assemblies comprising othermaterials). The term “semiconductor structure” used herein (including inthe claims) may refer to shallow trench isolation features, poly-silicongates, lightly doped drain regions, doped wells, contacts, vias, metallines, or other types of circuit patterns or features to be formed on asemiconductor substrate. In addition, the term “semiconductor structure”used herein (including in the claims) may refer to various semiconductordevices, including transistors, capacitors, diodes, and other electronicdevices that obey semiconductor physics and/or quantum mechanics.

What is claimed is:
 1. A method for annealing a semiconductor structure,the method comprising: providing a semiconductor structure; providing anenergy-converting material capable of increasing the semiconductorstructure's absorption of microwave radiation; providing a heatreflector between the energy-converting material and the semiconductorstructure, the heat reflector being capable of reflecting thermalradiation from the semiconductor structure; and applying microwaveradiation to the energy-converting material and the semiconductorstructure to anneal the semiconductor structure for fabricatingsemiconductor devices.
 2. The method of claim 1, wherein the heatreflector includes a semiconductor wafer.
 3. The method of claim 1,wherein the thermal radiation includes infrared radiation.
 4. The methodof claim 3, wherein the heat reflector has a reflectivity larger than95% with respect to infrared radiation.
 5. The method of claim 1,wherein the heat reflector is approximately transparent to the microwaveradiation.
 6. The method of claim 1, wherein the energy-convertingmaterial is capable of increasing an electric field density associatedwith the semiconductor structure.
 7. The method of claim 6, wherein theenergy-converting material increases the electric field density inresponse to the microwave radiation to further activate dopants throughinterfacial polarization.
 8. The method of claim 6, wherein: thesemiconductor structure includes one or more dopant clusters; and thesemiconductor structure's absorption of microwave radiation is increasedin response to the increased electric field density so as to dissolvethe dopant clusters.
 9. The method of claim 1, wherein the heatreflector is disposed at a distance from the semiconductor structure.10. The method of claim 9, wherein the distance is adjusted to improvethe heat reflector's reflection of thermal radiation from thesemiconductor structure.
 11. The method of claim 1, wherein the heatreflector is disposed at a distance from the energy-converting material.12. The method of claim 1, wherein the heat reflector is formed on theenergy-converting material.
 13. The method of claim 1, wherein theenergy-converting material includes boron-doped silicon germanium,silicon phosphide, titanium, nickel, silicon nitride, silicon dioxide,silicon carbide, n-type doped silicon, or aluminum capped siliconcarbide.
 14. The method of claim 1, further comprising: providing asecond energy-converting material so that the semiconductor structure isdisposed between the heat reflector and the second energy-convertingmaterial, the second energy-converting material being capable ofincreasing the semiconductor structure's absorption of microwaveradiation.
 15. The method of claim 1, wherein the microwave radiationhas a frequency within a range of approximately 2 GHz to approximately10 GHz.
 16. The method of claim 1, wherein the energy-convertingmaterial has a loss tangent in a range of approximately 0.01 toapproximately
 2. 17. A system for annealing a semiconductor structure,comprising: an energy-converting material configured to increase asemiconductor structure's absorption of microwave radiation; a heatreflector configured to reflect thermal radiation from the semiconductorstructure, the heat reflector being disposed between theenergy-converting material and the semiconductor structure; and amicrowave-radiation source configured to apply microwave radiation tothe energy-converting material and the semiconductor structure to annealthe semiconductor structure for fabricating semiconductor devices. 18.The system of claim 17, wherein the thermal radiation includes infraredradiation.
 19. The system of claim 17, wherein the heat reflectorincludes a semiconductor wafer.
 20. The system of claim 17, wherein theheat reflector is approximately transparent to the microwave radiation.